I agree with Mike. Every time you import your workouts, the RA remembers the most recent workout's time. The next time you import, the importer compares the last imported workout's time against the list of files it gets back from the Garmin Communicator.

It looks like the plugin is returning file modification file. Sometimes the file time is out of sync with the actual workout time. For example, let's say you started your run at 2014-04-01T09:00:00Z and finished at 2014-04-01T10:00:00Z, the file time might be 2014-04-01T10:35:00Z. I'm guessing something touched the file (perhaps the Ant Agent)? The next time you import, this workout will be imported again because the Communicator plugin is reporting a more recent time.